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Red blood cells, computer artwork. Red blood cells, or erythrocytes, are biconcave disc-shaped cells that are responsible for supplying tissues with oxygen. They circulate in the bloodstream, transporting oxygen from the lungs to the body, and returning carbon dioxide to the lungs for exhalation. Their red colour comes from an iron- rich, oxygen-binding protein called haemoglobin. Unlike most cells, they have no nucleus, which maximises their oxygen-carrying capacity. They are about 7 micrometres in diameter.